1. Field of the Invention
The present invent ion relates to an automatic exposure control apparatus employed for a color video camera, etc.
2. Description of the Prior Art
In FIG. 1, portions other than the portions represented by the numerals 215, 216, 217, 218, 219, 220, 221 and 222 correspond to a prior art. The prior art will be described. Light having passed through a taking lens 201 is directed to an image sensor device 204 such as a CCD, etc. after its light quantity being settled by an iris 202. After amplified by an amplifier 205, a output signal of the image sensor device 204 is detected by a detecting circuit 212. The iris 202 is driven by an iris driving portion 213 so that the level of the detecting output is constant, that is, so that the CCD (sensor device 204) is at an optimal operating point.
After amplified by a variable gain amplifier 206, a signal output from an image sensor device 204 is divided in two directions. One is directed to a color separating circuit 207, where a color signal which is coated with a color filter attached to the image sensor device 204 is separated, and is color-signal-processed at a color signal processing circuit 208 to be converted into color difference signals R-Y and B-Y. The other is directed to a luminance signal processing circuit 210, where a luminance signal component thereof is luminance-signal-processed, and is inputted to an encoder circuit 209. The color difference signals of the former are balanced-modulated by the encoder circuit 209 and added with a luminance signal to be converted into a composite video signal 214. The output of the luminance signal processing circuit is detected by a detecting circuit 211 and returned to the variable amplifier 206 to settle a gain of the variable gain amplifier 206 so that the level of the output of the detecting circuit 211 is constant.
Final level of the video signal is determined by the variable gain amplifier 206. At lower illumination, since the level of a luminance signal is low even if an iris 202 is opened to its maximum value, a gain of the variable gain amplifier 206 is increased so that the level of the luminance signal becomes constant.
As described above, exposure has been conventionally controlled not according to a color signal but by controlling a luminance signal level so as to be constant.
Other than the above-described example, some conventional automatic exposure control apparatuses detect an object position by a difference among luminances or high-frequency components inside and outside of a photometry area within a shooting image plane to automatically follow the photometry area (see Japanese laid-open Patent Application No. H1-120178 and Japanese laid-open Patent Application No. H1-120181).
However, since none of the above-described control apparatuses sets exposure considering what is a main object, an exposure level is sometimes inappropriate particularly when a person is a main object.
When exposure of a video camera is manually set, it is well-known that a desirable reproduction is obtained if exposure is set: (1) so that a signal level of a highlighted portion in an image plane is 100IRE (Institute of Radio Engineers); and (2) so that a signal level of a color of person's skin is between 55IRE to 75IRE.
As described above, an appropriate exposure level differs according to whether a person is a main object or not. Hence, with the conventional exposure controlling method, a signal level of a person's skin is not always appropriate even if a luminance signal level in an image plane is constant, particularly when a person is backlit or a when the background is dark and a person is highlighted.
In order to obtain an appropriate exposure level when a person is a main object, it is considered that a skin-colored portion is extracted to make a signal level thereof constant.
However, even if there is a skin-colored portion, the skin-colored portion is not always a main object. Consequently, although the skin-colored portion is adequately exposed, the entire image plane is sometimes inadequately exposed, for example, when a person shot is located in a corner, when a person passes across an image plane, or when there are a plurality of persons.
Moreover, even if a skin color is detected in an image plane, an appropriate exposure level differs according to whether the person is a main object or not.